Charged particle beam apparatus and automatic astigmatism adjustment method

ABSTRACT

According to the invention, techniques for automatically adjusting for astigmatism in a charged particle beam apparatus. Embodiments according to the present invention can provide a charged particle beam apparatus and an automatic astigmatism adjustment methods capable of automatically correcting astigmatism and a focal point in a relatively short period of time by finding a plurality of astigmatism correction quantities and a focal point correction quantity in a single operation from a relatively small number of 2 dimensional images. Specific embodiments can perform such automatic focusing while minimizing damages inflicted on subject samples. Embodiments include, among others, a charged particle optical system for carrying out an inspection, a measurement and a fabrication with a relatively high degree of accuracy by using a charged particle beam.

CROSS-REFERENCES TO RELATED APPLICATIONS

This application is a continuation application and claims priority ofU.S. patent application Ser. No. 10/980,096, filed on Nov. 2, 2004,which is a continuation of U.S. application Ser. No. 09/603,459, filedon Jun. 22, 2000 which claims priority from Japanese Patent ApplicationReference No. 11-176681, filed Jun. 23, 1999 all of which areincorporated by reference.

BACKGROUND OF THE INVENTION

The present invention relates generally to a charged particle beamapparatus and specifically to techniques for automatically adjusting forastigmatism.

An electron microscope is used as an automatic inspection system forinspecting and measuring a fine circuit pattern created on a substratesuch as a semiconductor wafer. In the case of a defect inspection, anelectron beam image detected by a scanning electron microscope isacquired and compared with a reference image used as a comparisonstandard. In a measurement of a hole diameter or a fine circuit patternused in monitoring and setting a manufacturing process condition of asemiconductor device, measurement of a length is based on imageprocessing of an electron beam image detected from the scanning electronbeam microscope. In an inspection to detect a defect of a pattern bycomparison of an electron beam image of the pattern with a referenceimage and in a measurement of a line width of a pattern by processing anelectron beam image, the quality of the electron beam image greatlyaffects the reliability of the result of the inspection and themeasurement.

The quality of an electron beam image deteriorates due to causes such asastigmatism of the electron beam system and degradation of theresolution attributed to defocusing. A poor quality electron beam imagecauses the inspection sensitivity and the performance of the lengthmeasurement to deteriorate. In addition, in such an image, the patternwidth varies and a result of detection of an image edge can not beobtained in a relatively stable manner. In consequence, results ofmeasuring the wire width of a pattern and the diameter of a hole withsuch a poor quality beam will often be unsatisfactory.

Conventionally, the focal point and the astigmatism of an electron beamoptical system are adjusted by properly changing a control current of anobjective lens and control currents of 2 coil sets each comprising aplurality of astigmatism correction coils while visually observing anelectron beam image. To be more specific, the focal point is adjusted byvarying the convergence height of a beam. The convergence height of abeam is changed by adjusting a current flowing through the objectivelens.

While there are perceived advantages, it can take time to execute theconventional technique of adjusting a control current of an objectivelens and control currents of 2 coil sets each comprising a plurality ofastigmatism correction coils while visually observing an electron beamimage as described above. In addition, the conventional techniques oftenrequire that the surface of a sample be scanned by using an electronbeam several times. As a result, it is quite within the bounds ofpossibility that a problem of a damage inflicted on the sample arises.In addition, since in conventional systems, adjustments are oftencarried out manually, the result of the adjustment varies from operatorto operator. Moreover, the astigmatism and the focal position can changewith time. It is thus necessary to adjust the astigmatism and the focalposition periodically by manual operations in order to carry out anautomatic inspection and an automatic measurement of a length.

What is needed are automated techniques for controlling electron beams.

SUMMARY OF THE INVENTION

According to the invention, techniques for automatically adjusting forastigmatism in a charged particle beam apparatus are provided.Embodiments according to the present invention can provide a chargedparticle beam apparatus and an automatic astigmatism adjustment methodscapable of automatically correcting astigmatism and a focal point in arelatively short period of time by finding a plurality of astigmatismcorrection quantities and a focal point correction quantity in a singleoperation from a relatively small number of 2 dimensional images.Specific embodiments can perform such automatic focusing whileminimizing damages inflicted on subject samples. Embodiments include,among others, a charged particle optical system for carrying out aninspection, a measurement and a fabrication with a relatively highdegree of accuracy by using a charged particle beam.

Numerous benefits are achieved by way of the present invention overconventional techniques. The present invention can provide specificembodiments with the capability of automatically adjusting astigmatismand a focal point at a relatively high speed and with a relatively highdegree of precision by using a small number of particle images obtainedas a result of radiation of a converged charged particle beam to asample in a scanning operation without inflicting a damage on thesample. In addition, select embodiments according to the presentinvention can also provide the capability of carrying out an automaticinspection of a pattern defect, such as a foreign substance on an objectsubstrate, or an automatic measurement of dimensions of a pattern on theobject substance. In specific embodiments, such tasks can be carried outwith a relatively high degree of precision while sustaining the qualityof a particle image in a relatively stable manner over a relatively longperiod of time by using the particle image. The particle image isobtained as a result of radiation of a converged charged particle beamwhile adjusting astigmatism and focal point thereof automatically and ata relatively high speed and with a relatively high degree of precision.

Select embodiments according to the present invention can provide acharged particle beam apparatus and an automatic astigmatism adjustmentmethod capable of automatically correcting astigmatism in a relativelyshort period of time for a variety of samples. Such embodimentsaccording to the invention can find a plurality of astigmatismcorrection quantities at the same time from a small number of 2dimensional images. In specific embodiments, damage inflicted on samplescan be kept to a minimum.

Furthermore, some specific embodiments according to the presentinvention can provide a charged particle beam apparatus capable ofcarrying out inspections, measurements and fabrications with arelatively high degree of reliability and in a relatively stable mannerover a relatively long period of time. The quality of a charge particleimage obtained from an object substrate as a result of automaticcorrection of astigmatism and a focal point of a charged particle beamoptical system is improved in some embodiments.

Moreover, many specific embodiments according to the present inventioncan provide a sample used for an adjustment of astigmatism and a focalpoint of a charged particle beam and suitable for an automaticcorrection of the astigmatism and the focal point in a relatively shortperiod of time by suppressing damages inflicted on the sample to aminimum in a charged particle beam optical system.

Yet further, some specific embodiments according to the presentinvention can provide an automatic astigmatism adjustment method capableof automatically correcting astigmatism and a focal point in arelatively short period of time from a 2 dimensional particle image andto provide a sample for the method.

These and other benefits are described throughout the presentspecification. A further understanding of the nature and advantages ofthe invention herein may be realized by reference to the remainingportions of the specification and the attached drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates a diagram showing a representative configuration ofan inspection and measurement apparatus in an example charged particlebeam apparatus provided by a particular embodiment of the presentinvention;

FIG. 2 illustrates an explanatory diagram showing astigmatism correctioncoils in a particular embodiment according to the present invention;

FIG. 3 illustrates a diagram showing a relation of astigmatism andshapes of a beam spot;

FIG. 4 illustrates a diagram showing representative patterns used forcorrecting astigmatism and a focal point in a particular embodiment ofthe present invention;

FIG. 5 illustrates a flowchart of representative image processingexecuted by an astigmatism and focal point correction quantitycomputation circuit in a particular embodiment according to the presentinvention;

FIG. 6 illustrates diagram showing a relation among a computeddirectional sharpness magnitude, an astigmatism magnitude, anastigmatism direction and a focal offset z;

FIG. 7 illustrates a diagram showing representative image processing inorder to find directional sharpness magnitudes in a particularembodiment according to the present invention;

FIG. 8 illustrates a diagram showing shapes of a calibration target or asample for calibration of astigmatism and a focal point at a relativelyhigh speed;

FIG. 9 illustrates a flowchart of representative image processingexecuted by the astigmatism and focal point correction quantitycomputation circuit employed in the charged particle beam apparatusshown in FIG. 1 and using the calibration target shown in FIG. 8 in aparticular embodiment according to the present invention;

FIG. 10 illustrates a diagram showing a representative visual fieldmovement sequence for periodically calibrating drifts of astigmatism anda focal point in a particular embodiment according to the presentinvention;

FIG. 11 illustrates a diagram showing a technique of interpolation of amaximum value position of a curve representing the directional sharpnessmagnitude in a particular embodiment according to the present invention;

FIG. 12 illustrates an explanatory diagram showing a case in which acurve representing the directional sharpness magnitude exhibits a doublepeaked characteristic in a particular embodiment according to thepresent invention;

FIG. 13 illustrates a diagram showing a technique of finding a center ofgravity as a center position of a curve representing the directionalsharpness magnitude in a particular embodiment according to the presentinvention;

FIG. 14 illustrates a diagram showing a technique of computing aweighted average of positions of maximum values as a center position ofa curve representing the directional sharpness magnitude in a particularembodiment according to the present invention; and

FIG. 15 illustrates a diagram showing a technique of finding a centerposition of a curve representing the directional sharpness magnitude bysymmetry matching in a particular embodiment according to the presentinvention.

DESCRIPTION OF THE SPECIFIC EMBODIMENTS

The present invention provides techniques for automatically adjustingfor astigmatism in charged particle beam apparatus. Embodimentsaccording to the present invention can provide a charged particle beamapparatus and automatic astigmatism adjustment techniques capable ofautomatically correcting astigmatism, as well as focal point, in arelatively short period of time. Embodiments can find a plurality ofastigmatism correction quantities and a focal point correction quantityin a single operation from a relatively small number of 2 dimensionalimages.

A variety of automatic astigmatism correction techniques have beenproposed. According to one technique, a secondary electron signalobtained from a sample as a result of 2 dimensional scanning over thesample by using a charged particle beam is differentiated to extractdigital data with a large variation. Then, a position on the samplecorresponding to the extracted digital data is found. The chargedparticle beam is then used for scanning the sample only in the X and Ydirections with the found position taken as a center while an excitationcurrent flowing through the an objective lens is being changed. Then,focal information in the X direction and focal information in the Ydirection are detected at a maximum value of the digital data of thesecondary electron signal obtained as a result of the scanning in eachof these directions. From the focal information in the X direction andthe focal information in the Y direction, the magnitude of a currentflowing through the objective lens is determined and the current of thedetermined magnitude is output to the objective lens. Later, a currentflowing through an astigmatism correction coil is changed and a chargedparticle beam is used for scanning in one direction, namely, the X or Ydirection, and the magnitude of the current flowing through theastigmatism correction coil is determined at a maximum value of thedigital data of a secondary electron signal obtained as a result of thescanning to adjust the focus and the astigmatism of the output chargedparticle beam. For a more detailed description of such techniques,reference may be had to a Japanese Patent Prepublication No. Hei7-153407.

According to another technique, while an electron beam is being used inscanning in a variety of directions, a focal point is vibrated to findthe direction of astigmatism. Then, while a relation between 2astigmatism quantities of different types is being sustained so that theastigmatism changes only in a particular direction, the astigmatismquantities are changed to search for a condition producing a clearimage. Thus, conditions of the astigmatism quantities of 2 freedoms canbe adjusted by limiting the quantities to 1 freedom, allowing theadjustment time to be shortened. For a more detailed description of suchtechniques, reference may be had to a Japanese Patent Prepublication No.Hei 9-161706.

In a yet further technique, after a focal point is adjustedautomatically to make a shift from an in focus state to a state slightlyshifted away from the in focus state. Then, the direction of astigmatismis found by applying FFT to a 2 dimensional image. Then, while arelation between 2 astigmatism quantities of different types is beingsustained so that the astigmatism changes only in this direction, theastigmatism quantities are changed to search for a condition producing aclear image. For a more detailed description of such techniques,reference may be had to a Japanese Patent Prepublication No. Hei10-106469.

In a still yet further technique, a point at which a change of a Fouriertransformation quantity is inverted is found by changing the focal pointby application of Fourier transformation of a 2 dimensional image. Then,a secondary electron image at a focal point before an in focus state anda secondary electron image at a focal point after the in focus state aremeasured. The direction of astigmatism is then found from distributionsof the maximums and the minimums. Then, the astigmatism is corrected sothat the astigmatism changes in this direction. For a more detaileddescription of such techniques, reference may be had to a in JapanesePatent Prepublication No. Hei 9-82257.

In a still yet further technique, an astigmatism correction quantity canbe determined using sharpness magnitudes in 4 directions of scanningelectron microscope images acquired by increasing the position of thefocal point are found. Then, the position of the focal point is furtherincreased till a maximum value of each of the sharpness magnitudes isobtained. Finally, the astigmatism correction quantity is found from themaximum values of the sharpness magnitudes in these 4 directions. Byusing this technique, an astigmatism correction quantity can be foundfrom a series of scanning electron microscope images with their focalpoints shifted from each other. In this technique, however, maximumvalues of sharpness magnitudes are used and whole changes in sharpnessmagnitude are not taken into consideration. For a more detaileddescription of such techniques, reference may be had to U.S. Pat. Ser.No. 6,025,600.

Techniques that find a point at which the sharpness of a particle imagereaches a maximum by using a trial and error technique while changing atotal of 3 control quantities, namely, 2 astigmatism correctionquantities of different types and a focal point correction quantity, cantakes a relatively long time to complete the correction process. If thisoccurs, a charged particle beam is radiated to the sample for a longtime, resulting in damage to the sample due to electric charge and dirtaccumulated in the sample. In addition, when an automatic or visualadjustment is carried out by using the sharpness as a measure, there ismore likely a case in which a condition can not be set to makeastigmatism truly disappear in dependence on a pattern on the sample.

Techniques that find a direction of astigmatism by vibrating the focalpoint, and then perform a 1 dimensional scanning operation repeatedly byvibrating the focal point while the astigmatism correction quantity isbeing changed, may necessitate repeatedly carrying out an operation tosearch for a condition in which focal points in 2 directions coincidewith each other, raising a problem that it takes time to repeatedlycarry out such an operation. In addition, a 1 dimensional scanningoperations performed on a sample can leave a radiation mark on thesample. Furthermore, there is also raised a problem of stability in thecorrection of the astigmatism due to the fact that a signal with asufficient strength can not be obtained from some locations experiencinga 1 dimensional scanning operation because of non uniform texture on thesample.

Techniques in which a 2 step adjustment is carried out in order tovibrate an astigmatism correction quantity after a focal point isvibrated raise a issue of a long time to complete the adjustment. Also,an issue of severe damages inflicted on the sample can arise. Inaddition, the processing to find the direction of astigmatism by usingthe FFT can require a presumption of a uniform spectrum of an imageobtained with no astigmatism generated, imposing a limitation on usablesamples.

Many techniques do not determine a direction and a magnitude ofastigmatism from a particle image in a relatively stable manner, norcompute a correction quantity from the direction and the magnitude ofastigmatism to be adopted in an astigmatism adjustment device. Thus, theastigmatism correction quantity may need to be changed, which cannecessitate the correction quantity be examined repeatedly on a trialand error basis. As a result, it takes time to carry out the adjustmentand, damage can be inflicted on the sample due to contamination andbuildup of a charge on the sample. Furthermore, precision deteriorationcan occur for a location on the sample with a coarse pattern scanned ina 1 dimensional scanning operation.

Techniques in which the direction and the “strength” of astigmatism arefound by applying Fourier transformation to a 2 dimensional image with avibrating focal point, may not be able to find a correction quantity foran astigmatism adjustment device from the direction and the “strength”of astigmatism. There is thus raised a problem of an inability to find acorrection quantity for the astigmatism adjustment means with asufficiently high degree of accuracy.

Techniques that determine an astigmatism correction quantity usingsharpness magnitudes in 4 directions can fail to find the correctionquantity in instances where the curve representing the changes insharpness magnitude is an asymmetrical or has a double peaked character.In addition, an astigmatism correction quantity may not always be foundfrom a series of scanning electron microscope images with their focalpoints shifted from each other with a relatively high degree ofaccuracy. This is especially true in cases were the sharpness curvetends to be asymmetric or have double peaks, or when the astigmatism islarge.

In order to solve the problems of prior art techniques, such as anincreased number of errors generated in correction of astigmatism due tothe use of maximum values of directional sharpness magnitudes in ananalysis of the directional sharpness magnitudes, embodiments accordingto the present invention provide techniques for finding a center ofgravity of a curve representing the directional sharpness magnitude.Techniques based on a center of gravity can correct the center positionof an area under a asymmetrical or doubled peaked curve representing thedirectional sharpness magnitude toward a region with a wider base in thecase of the asymmetrical curve or a region including the auxiliary humpin the case of the double peaked curve. As a result, the astigmatism canbe corrected relatively surely as well as relatively accurately.

Astigmatism correction quantities can include error due to an effect ofaberration other than astigmatism of the charged particle opticalsystem. Thus, if the magnitude of astigmatism is large, the astigmatismis corrected repeatedly a plurality of times if necessary till thechange in astigmatism correction quantity is reduced or converged to asmall value. By this way, the failure in correcting astigmatism can beprevented.

Specific embodiments of the present invention that implement a chargedparticle beam apparatus, automatic astigmatism correction techniques andsamples for adjustments of astigmatism of a charged particle beam areexplained by referring to diagrams.

FIG. 1 illustrates an inspection and measurement apparatus in a chargedparticle beam apparatus in a particular embodiment according to thepresent invention. FIG. 1 illustrates a charged particle optical system10, a control system for controlling a variety of elements comprisingthe charged particle optical system 10 and an image processing systemfor processing an image on the basis of secondary particles or reflectedparticles detected by a particle detector 16 in the charged particleoptical system 10.

The charged particle optical system 10 comprises a charged particle beamsource 14 for emitting a charged particle beam such an electron beam oran ion beam and an astigmatism corrector 60 for correcting astigmatismof a charged particle beam emitted by the charged particle beam source14 by providing an electric field. Further, a beam deflector 15 fordeflecting the charged particle beam emitted by the charged particlebeam source 14 in a scanning operation can also be part of chargedparticle optical system 10. An objective lens 18 for converging thecharged particle beam deflected by the beam deflector 15 by means of amagnetic field; a sample base 21 for mounting a sample 20 and fixing acalibration target 62 beside the sample 20; an XY stage 46 for mountingand moving the sample base 21; a grid electrode 19 set at an electricpotential close to that of the ground can also be included in system 10.Further, a retarding electrode (not shown in the figure) can be used forproviding a negative electric potential relative to the sample 20 andthe calibration target 62 mounted on the sample base 21 to an electronbeam radiated by the charged particle beam source 14 as a chargedparticle beam or a positive electric potential relative to the sample 20and the calibration target 62 to an ion beam radiated by the chargedparticle beam source 14 as a charged particle beam. FIG. 1 alsoillustrates a height detection sensor 13 for measuring the height of thesample 20 or another object; and the particle detector 16 for detectingsecondary particles or reflected particles which emanate from thesurface of the sample 20 as a result of radiation of a charged particlebeam to the sample 20 and are reflected typically by a reflective plate.

It should be noted that the astigmatism corrector 60 can be implementedby an astigmatism correction coil based on a magnetic field or anastigmatism correction electrode based on an electric field. Theobjective lens 18 can be implemented by an objective coil based on amagnetic field or a static objective lens based on an electric field. Inaddition, the objective lens 18 can also be provided with a coil 18 afor correction of the focal point. An astigmatism adjustment meanscomprises the astigmatism corrector 60 and an astigmatism correctioncircuit 61.

A stage control unit 50 drives and controls the movement or the travelmotion of the XY stage 46 while monitoring the position or thedisplacement of the XY stage 46 on the basis of a control command issuedby a whole control unit 26. It should be noted that the XY stage 46 isprovided with a position monitoring length measurement unit formonitoring the position or the displacement of the XY stage 46. Themonitored position or displacement of the XY stage 46 can be supplied tothe whole control unit 26 through the stage control unit 50.

A focal position control unit 22 drives and controls the objective lens18 in order to adjust the focal point of the charged particle beam to aposition on the sample 20 on the basis of height information of thesurface of the sample 20 measured by a height detection sensor 13 inaccordance with a command issued by the whole control unit 26. It shouldbe noted that, by adding a Z stage to the XY stage 46, the focal pointcan be adjusted by driving and controlling the Z stage in place of theobjective lens 18. The objective lens 18 or the Z stage, the focalposition control unit 22 and other components constitute a focus pointcontrol means.

A deflection control unit 47 supplies a deflection signal to the beamdeflector 15 in accordance with a control command issued by the wholecontrol unit 26. At that time, a correction is added to the deflectionsignal so as to compensate for a variation in image magnificationaccompanying a change in surface height of the sample 20 and for animage rotation accompanying control of the objective lens 18.

A grid potential adjustment unit 48 adjusts the close to the groundelectric potential applied to the grid electrode 19 provided at aposition above and close to the sample 20 in accordance with a potentialadjustment command issued by the whole control unit 26. A sample basepotential adjustment unit 49 adjusts an electric potential applied tothe retarding electrode provided above the sample base 21 in accordancewith a potential adjustment command issued by the whole control unit 26.A negative or positive electric potential applied to the sample 20 bythe grid electrode 19 and the retarding electrode reduces the velocityof an electron beam or an ion beam traveling between the objective lens18 and the sample 20 in order to raise the resolution in a lowacceleration voltage area.

A beam source potential adjustment unit 51 adjusts an electric potentialapplied to the charged particle beam source 14 in accordance with acommand issued by the whole control unit 26 to regulate a beam currentor an acceleration voltage of a charged particle beam emitted by thecharged particle beam source 14.

The beam source potential adjustment unit 51, the grid potentialadjustment unit 48 and the sample base potential adjustment unit 49 arecontrolled by the whole control unit 26 so that a particle image with adesired quality can be detected by the particle detector 16.

In processing to correct astigmatism and a focal point, an astigmatismadjustment unit 64 provided by the present invention issues a controlcommand to change the focal point (also referred to as a focus f) to thefocal position control unit 22. Receiving this command, the focalposition control unit 22 drives and controls the objective lens 18 so asto change the focus f of a charged particle beam radiated to an area onthe sample 20 or the calibration target 62 with a certain patterncreated therein. The pattern typically includes edge components of aboutthe same quantity in directions shown in FIGS. 4 (a) and (b). By doingso, the particle detector 16 detects a plurality of particle imagesignals with different loci f. Each of the particle image signals isconverted by an A/D converter 24 into a particle digital image signal ordigital image data to be stored in an image memory 52, being associatedwith a respective focus command value f output by the astigmatismadjustment unit 64. Then, an astigmatism and focus correction quantitycomputation image processing circuit 53 reads out the particle digitalimage signals with different foci f from the image memory 52, findingdirectional sharpness magnitudes d0 (f), d 45 (f), d90 (f) and d135 (f)for each of the particle digital image signals. The astigmatism andfocus correction quantity computation image processing circuit 53 thenfinds focus values f0, f45, f90 and f135 that generate peak values ofthe directional sharpness magnitudes d0 (f), d 45 (f), d90 (f) and d135(f) respectively. Then, the astigmatism and focus correction quantitycomputation image processing circuit 53 finds the astigmatism (to bemore specific, an astigmatism vector (dx and dy) or an astigmatismdirection α and an astigmatism magnitude δ) and a focal offset value zfrom the focus values f0, f45, f90 and f45, supplying the astigmatismand the focal offset value z to the whole control unit 26 to be storedin a storage unit 57. From a relation between the astigmatism and theastigmatism correction quantity found in advance, the whole control unit26 computes astigmatism correction quantities Δstx and Δsty for theastigmatism which is found as described above and stored in the storageunit 57. It should be noted that the relation between the astigmatismand the astigmatism correction quantity represents a characteristic ofthe astigmatism corrector 60. By the same token, from a relationrepresenting a characteristic of the objective lens 18 found in advance,the whole control unit 26 computes a focal point correction quantity forthe focal offset value z found as described above and stored in thestorage unit 57. The astigmatism correction quantities Δstx and Δsty andthe focal point correction quantity found in this way are supplied tothe astigmatism adjustment unit 64.

The astigmatism adjustment unit 64 passes on the astigmatism correctionquantities Δstx and Δsty received from the whole control unit 26 to theastigmatism adjustment circuit 61 so as to allow the astigmatismcorrector 60 to correct the astigmatism of the charged particle beam.The astigmatism corrector 60 is an astigmatism correction coil based ona magnetic field or an astigmatism correction electrode based on anelectric field. By the same token, the astigmatism adjustment unit 64passes on the focal point correction quantity to the focal positioncontrol unit 22 to control a coil current flowing to the objective lens18 or a coil current flowing to a focal point correction coil 18 a. As aresult, the focal point is corrected.

As another technique, the XY stage 46 is provided with a Z stage asdescribed above. In this case, the astigmatism adjustment unit 64outputs a control command to vibrate the focal point or to change thefocus to the stage control unit 50 through the whole control unit 26 ordirectly. Receiving this command, the stage control unit 50 vibrates thefocal point by driving a Z shaft of the XY stage 46. Thus, a particleimage with a vibrating focal point is obtained from the particledetector 16. In the astigmatism and focus correction quantitycomputation image processing circuit 53, astigmatism and focal pointcorrection quantities are found and the computed focal point correctionquantity is fed back to the Z shaft of the XY stage 46 to allow acorrection to be carried out. Of course, a component for acquiring animage with a vibrating focal point and a component subjected to thefinal focal point correction can be provided separately. To put itconcretely, one of them is implemented by the focal position controlunit 22 while the other is implemented by the Z shaft of the XY stage46. As another alternative, the two components are combined to executecontrol by using both at the same time. In either case, the focalposition relative to the location of the sample 20 or the calibrationtarget 62 needs to be controlled to a desired distance. It should benoted that the technique of controlling the objective lens 18 offers aresponse characteristic superior to that of the technique of controllingthe Z shaft in at least one specific embodiment.

As described above, the correction of the astigmatism and the focalpoint is based on control executed by the astigmatism adjustment unit 64in accordance with a command issued by the whole control unit 26. As aresult, the whole control unit 26 receives a particle image with theastigmatism and the focal point thereof corrected from the image memory52 directly or through the astigmatism and focus correction quantitycomputation image processing circuit 53 and displays the particle imageon a display means 58 to allow the user to visually form a judgment asto whether the correction of the astigmatism and the like is correct orincorrect.

Furthermore, in an inspection or a measurement, the XY stage 46 iscontrolled to take a predetermined position on the sample 20 to thevisual field of the charged particle optical system. Then, a particleimage signal obtained by the particle detector 16 is converted by theA/D converter 24 into a particle digital image which is then stored inan image memory 55. Subsequently, an inspection and measurement imageprocessing circuit 56 measures dimensions of a fine pattern created onthe sample 20 or inspects the sample 20 for a defect of a fine patternor a defect such as an infinitesimal foreign material on the basis of adetected particle digital image signal stored in the image memory 55.Results of the measurement and the inspection are supplied to the wholecontrol unit 26. At that time, by correcting the astigmatism and thefocal point in accordance with the technique provided by the presentinvention at least periodically, it is possible to implement ameasurement and an inspection based on a particle image with theastigmatism always corrected.

It should be noted that, in an inspection of a defect or the like basedon a particle image, the inspection and measurement image processingcircuit 56 may generate a reference particle digital image signal to beused as a comparison object by delaying a detected detection particledigital image signal and then comparing a current detection particledigital image signal with the reference particle digital image at aposition corresponding to that of the current detection particle digitalimage signal to detect a mismatch or a difference between the 2 signalsas a defect candidate. Then, the inspection and measurement imageprocessing circuit 56 carries out processing to recognize acharacteristic quantity of each defect candidate and form a judgment asto whether or not to eliminate false information from characteristicquantities in order to inspect the sample 20 for a real defect.

Slightly affected by things such as electrical charge and dirtaccumulated on the sample 20 and damage inflicted on the sample 20, theoptical height detection sensor 13 is capable of detecting variations inheight of the sample 20 at positions being measured or inspected. Thevariations are fed back to the focal position control unit 22 so that anin focus state can be sustained all the time. When the optical heightdetection sensor 13 is used in this way, the astigmatism and the focalposition can be automatically adjusted at another position on the sample20 or at the calibration target 62 provided on the sample base 21 inadvance or periodically in the course of a measurement or an inspection.Thus, radiation of a converged charged particle beam for an automaticadjustment of the astigmatism and the focal point can be omitted orreduced considerably, allowing effects of electric charge and dirtaccumulated on the sample 20 and a damage inflicted on the sample 20 tobe eliminated.

The following description explains automatic adjustment of theastigmatism and the focal point in a converged charged particle opticalsystem provided by a specific embodiment according to the presentinvention. In this specific embodiment according to the presentinvention, the amount of astigmatism and the focal offset are found froma small number of 2 dimensional particle images. The amount ofastigmatism and the focal offset are then converted into correctionquantities of the astigmatism and the focal offset respectively at thesame time in a one time correction process.

FIG. 2 is a diagram showing a configuration of 2 coil sets eachcomprising a plurality of coils. Based on magnetic fields, these 2 coilsets serve as an embodiment of the astigmatism corrector 60. In theconfiguration of the 2 coil sets each comprising a plurality ofastigmatism correction coils, a current flowing through one of the coilsets has an effect of expanding a beam in a certain direction whileshrinking the beam in a direction perpendicular to the certaindirection. If the 2 coil sets are controlled respectively by acombination of 2 magnetic fields stx and sty shifted from each other bya phase difference of 45 degrees as shown in FIG. 2, the astigmatism canbe adjusted in any arbitrary direction by a necessary amount. Of course,the astigmatism corrector 60 can also be designed to comprise electrodesbased on magnetic fields.

Next, states of astigmatism are explained by referring to FIG. 3. Acolumn on the left side represents states of converged charged particlebeams with astigmatism thereof corrected. A state on the top of a columnis a case with a high focal position (Z>0). A state in the middle of acolumn represents an in focus state (Z=0). A state at the bottom of acolumn is a case with a low focal position (Z<0). As indicated by thecolumn on the left side, at the in focus position, the charged particlebeam is converged on a small point. At positions above and below the infocus position, the diameter of the beam increases symmetrically withrespect to the in focus position. The column in the middle of FIG. 3represents states obtained as a result of flowing an stx current. ForZ>0, the beam is expanded in the horizontal direction. For Z<0, on theother hand, the beam is expanded in the vertical direction. At the infocus position, the beam forms a true circle but the diameter of thecircle is not reduced to a sufficiently small value. The column on theright side of FIG. 3 represents states obtained as a result of flowingan sty current. At the positions shifted from the in focus position, theorientation of elliptical beam is rotated by 45 degrees in directionsopposite to each other so that the direction of the major axis of theellipse for Z>0 is perpendicular to that for Z<0. By combining an stxcurrent with an sty current, astigmatism of any arbitrary direction canbe generated in any arbitrary direction so as to cancel pre-adjustmentastigmatism of the charged particle optical system. As a result, theastigmatism can be corrected.

As shown in FIG. 3, in a state with generated astigmatism, at a positionshifted from the in focus position, the charged particle beam blurs intoan elliptical shape. To be more specific, at +Z and −Z positionssandwiching the focal point, the elliptical shapes of the beam arethinnest and the major axes of the ellipses are oriented in directionsperpendicular to each other. The magnitude of the astigmatism isrepresented by the focal distance 2Z between the 2 shifted points andthe direction of the astigmatism is represented by the direction of theellipse. The focal distance 2Z between the 2 shifted points is referredto astigmatism which is denoted by notation δ shown in FIG. 6. On theother hand, the direction of the astigmatism is represented by a mainpoint main axis direction α shown in FIG. 6. An astigmatism vector canbe denoted by notation (dx, dy).

Next, correction of the astigmatism and the focal point is explained byreferring to FIGS. 4 to 7. FIGS. 4 (a) and (b) are diagrams each showingan embodiment of a pattern created on the sample 20 or the calibrationtarget 62 and used for correction of the astigmatism and the focalpoint. Any pattern can be used as a pattern for correction of theastigmatism and the focal point as long as the pattern includes edgecomponents of about the same quantity in at least 3 directions in whichastigmatism is generated. FIG. 4 (a) shows an embodiment with 4 patternsof straight lines created in different areas. The direction of thestraight lines varies from pattern to pattern. On the other hand, FIG. 4(b) shows curve shaped patterns which have edge components in 4directions and are laid out 2 dimensionally at a uniform pitch. In thecase of the sample 20, in particular, a pattern created thereon toinclude edge components of about the same quantity in at least 3directions is usable. In this case, however, it is necessary to supplyinformation on a position to create this pattern to the whole controlunit 26 in advance by using an input means 59 and store the informationtypically in the storage unit 57, or it is necessary for the operator tospecify a position on the sample 20 for each correction of theastigmatism and the focal position. In addition, it is a matter ofcourse that information on a position to install the calibration target62 on the sample base 21 is supplied to the whole control unit 26 inadvance by using the input means 59 and stored in the storage unit 57 inadvance.

The whole control unit 26 supplies the information on a position of apattern for correction of the astigmatism and the focal point to thestage controller 50. In accordance with the information, the stagecontroller 50 drives and controls the XY stage 46 to move the patternfor correction of the astigmatism and the focal point to a position inclose proximity to an optical axis of the charged particle opticalsystem.

(1) At a step S51 of a flowchart shown in FIG. 5, while the chargedparticle beam is being radiated to the pattern for correction of theastigmatism and the focal point in a scanning operation in accordancewith a command issued by the whole control unit 26 to the deflectioncontrol unit 47 and the focus f is being changed in accordance with acommand issued by the astigmatism adjustment unit 64 to the focalposition control unit 22, the particle detector 16 acquires a pluralityof images and stores them in the image memory 52. The astigmatism andfocus correction quantity computation image processing circuit 53computes directional sharpness magnitudes (at 0 degrees, 45 degrees, 90degrees and 135 degrees) as d0 (f), d45 (f), d90 (f) and d135 (f)respectively as shown in FIG. 6 (a). It should be noted that it ispossible to acquire a focus f as a value specified in a command issuedby the astigmatism adjustment unit 64 to the focal position control unit22.

(2) At a next step S52, the astigmatism and focus correction quantitycomputation image processing circuit 53 finds center positions p0, p45,p90 and p135 shown in FIG. 6 (a) for each curve representing the 4directional sharpness magnitudes as a function of focus f in one of thedirections.

(3) At a next step S53, the astigmatism and focus correction quantitycomputation image processing circuit 53 finds the direction α and themagnitude δ of a focal shift (or astigmatism) caused by directionalastigmatism from the center positions p0, p45, p90 and p135 on the basisof a sinusoidal function shown in FIG. 6 (b). The astigmatism and focuscorrection quantity computation image processing circuit 53 also findsthe focal offset z. The astigmatism direction α, the astigmatismmagnitude δ and the focal offset z are supplied to the whole controlunit 26 to be stored in the storage unit 57. It should be noted that, atthe step S53, the astigmatism vector (dx, dy) can also be found in placeof the direction α and the magnitude δ of the astigmatism. The magnitudeδ of the astigmatism can be expressed by Eq. (1) given below. On theother hand, the direction α of the astigmatism can be expressed by Eq.(2) given below. As for the focal offset z, Eq. (3) given below isapplicable.

$\begin{matrix}\begin{matrix}{\delta^{2} = {\left( {{p\; 0} - {p\; 90}} \right)^{2} + \left( {{p\; 45} - {p\; 135}} \right)^{2}}} \\{= {({dx})^{2} + ({dy})^{2}}}\end{matrix} & (1) \\\begin{matrix}{\alpha = {\left( {1/2} \right){\tan^{- 1}\left( {\left( {{p\; 45} - {p\; 135}} \right)/\left( {{p\; 0} - {p\; 90}} \right)} \right)}}} \\{= {\left( {1/2} \right){\tan^{- 1}\left( {({dy})/({dx})} \right)}}}\end{matrix} & (2) \\{z = {\left( {{p0} + {p\; 45} + {p\; 90} + {p\; 135}} \right)/4}} & (3)\end{matrix}$

It should be noted that the storage unit 54 is used for storing softwareincluding a program for finding the directional sharpness magnitudes d0(f), d45 (f), d90 (f) and d135 (f) described above, a program forfinding their center positions p0, p45, p90 and p135 from thedirectional sharpness magnitudes d0 (f), d45 (f), d90 (f) and d135 (f)respectively and a program for finding the astigmatism and the focaloffset. The astigmatism and focus correction quantity computation imageprocessing circuit 53 has a configuration capable of carrying outprocessing based on these programs. Of course, the storage unit 54 canbe implemented as a ROM or the like.

(4) At a next step S54, the whole control unit 26 becomes capable ofconverting or apportioning astigmatism (α and δ or (dx, dy)) into or torequired astigmatism correction quantities (1, 2) (Δstx, Δsty) by usinga relation between variations in astigmatism control values (stx, sty)and variation quantities (or sensitivities) in astigmatism direction aand astigmatism magnitude δ or in astigmatism vector (dx, dy). Thisrelation is a characteristic of the astigmatism corrector 60 which isfound in advance. At the next step S55, the whole control unit 26becomes capable of setting the astigmatism correction quantities (1, 2)(Δstx, Δsty) and the focal offset value z to be supplied to theastigmatism adjustment unit 64. It should be noted that the astigmatismcorrection quantities (1, 2) (Δstx, Δsty) and the focal value z may becomputed by the astigmatism and focus correction quantity computationimage processing circuit 53, which receives the characteristics of theastigmatism corrector 60 and the objective lens 18 from the wholecontrol unit 26 instead of being computed directly by the whole controlunit 26.

(5) The astigmatism adjustment unit 64 supplies the focal offset value zreceived from the whole control unit 26 to the focal position controlunit 22, which corrects an objective lens current flowing through theobjective lens 18 or a focus correction coil current flowing through thefocal point correction coil 18 a. The astigmatism adjustment unit 64also supplies the astigmatism correction quantities (Δstx, Δsty)received from the whole control unit 26 to an astigmatism correctioncircuit 61, which corrects an astigmatism correction coil current or anastigmatism correction static voltage. In this way, the correction ofthe astigmatism and the adjustment of the focal point can be carried outin a one time operation.

(6) In the case of a small amount of astigmatism, auto stigma processingis completed in the one time operation described above. In the case of alarge amount of astigmatism, on the other hand, the processing can notbe completed in a one time operation due to causes of aberration otherthan the astigmatism. Examples of aberration other than the astigmatismare high order astigmatism and image distortion. In this case, the flowof the procedure goes back to (1) to carry out the auto stigmaprocessing once again. The loop is executed repeatedly till the focaloffset z and the astigmatism correction quantities (Δstx, Δsty) arereduced to small values.

By adopting the technique described above, the astigmatism and the focalpoint can be adjusted at a relatively high speed in a one time operationwith only few small damages inflicted on the sample 20 and thecalibration target 62. In addition, while the focal point is beingchanged, the directional sharpness magnitudes of images of the samesample 20 or the same calibration target 62 are compared with each otherto find astigmatism. Thus, the astigmatism and the focal point can beadjusted with a relatively high degree of precision in a one timeoperation without relying on a pattern on the sample 20 or thecalibration target 62, that is, a pattern for correction of theastigmatism and the focal point. The only requirement for a patterncreated on the sample 20 or the calibration target 62 is that thepattern must include edge components of about the same quantity in eachdirection.

As described above, directional astigmatism magnitudes of 4 differenttypes, that is, astigmatism at θ=0 degrees, 45 degrees, 90 degrees and135 degrees, are used. It should be noted, however, that the angle θdoes not have to be the 4 directions provided that the astigmatismdirection α and the astigmatism magnitude δ are known. Directionalastigmatism magnitudes dθ(ƒ) at any arbitrary number of angles θ in atleast 3 directions can be used. For each angle θ, the center position pθof the curve dδ(ƒ) is found. Then, the amplitude and a phase of asinusoidal wave for the center position pθ are found as an astigmatismmagnitude δ and an astigmatism phase α respectively. It should be notedthat the waveform does not have to be sinusoidal. Instead, an almostsinusoidal waveform can be used.

The following description explains embodiments each used for findingdirectional sharpness magnitudes of a particle image in the astigmatismand focus correction quantity computation image processing circuit 53.

As a first embodiment, the particle detector 16 is used for detectingand observing a particle image by radiating a charged particle beam to asample (target) 62 for automatic astigmatism correction based on apattern with a direction varying from area to area as shown in FIG. 7(a) in a scanning operation. The directional sharpness magnitude dθ isfound by measuring the amplitude of a particle image in each of theareas. The amplitude can be found by calculating the difference betweenthe maximum value of s (x, y) and the minimum value of s (x, y) for eachof the areas, or by calculating the differential (V=Σ_(xy) (s (x,y)−smean)²/N) of the concentration value (or the gradation value) s (x,y) of the particle image for each of the areas. As an alternative, thesum Σ_(xy) |t (x, y)| where |t (x, y)| is the absolute value of adifferential t (x, y) of a 2 dimensional differential result s (x, y) oftypically Laplacian and the sum of squares Σ_(xy) (t (x, y)² are found.The sum found in this way is defined as a directional sharpnessmagnitude dθ. The angular direction θ can be defined in any arbitraryway. In the case of the example shown in the figure, the angulardirection θ is defined in a clockwise direction starting at 0 degreesfor the orientation of the direction of the normal line of the patterncoinciding with the horizontal direction. Directions of the pattern arenot restricted to the 4 directions shown in the figure. Instead, theangle range of 180 degrees can be divided into n almost equal segmentsand pattern directions can then be determined by possible combinationsof any of the n segments where n is an integer equal to or greater than3.

A second embodiment implements a sample 20 or a calibration target 62with a pattern like one shown in FIG. 7 (b), and finds the directionalsharpness magnitude dθ by calculating directional differentials for aparticle image detected by the particle detector 16. A directionaldifferentiation is implemented by carrying out convolution processingfor an image on a mask like one shown in the figure. Then, a sum ofsquares of values at points for an image obtained as a result of thedifferentiation is computed as a directional sharpness magnitude dθ. Itshould be noted that the differentiation mask shown in the figure istypical. That is, the use of this mask is not mandatory. Any mask can beused as long as a requirement for a mask used for computing directionaldifferentials is satisfied. The requirement is that 2 values at any 2positions symmetrical with respect to a certain axis have signs oppositeto each other but about equal magnitudes. There are a variety ofachievable differentiation mask variations for suppressing noise andimproving selectability of the differentiation direction. In addition,it is also necessary to select a filtering means prior to computation ofimage differentials and a means for shrinking the image. The selectedmeans are means suitable for the image. Furthermore, by carrying out adirectional differentiation after rotation of the image, a simple 0degree differentiation or a 90 degree differentiation can also beadopted to perform a directional differentiation at any arbitrarydirectional angle θ.

The following description explains an embodiment for finding the centerposition pθ for a directional sharpness magnitude expressed as afunction dθ(ƒ) of focal point f in the astigmatism and focus correctionquantity computation image processing circuit 53. As a technique forfinding the center position pθ, a proper method can be selected. Theselected method can be a technique for finding the center position pθ asa center position of a function such as a second order function or aGaussian function applied to values preceding and succeeding theposition of the focal point f giving a maximum value of the functiondθ(ƒ). Another selected method can be a technique for finding the centerposition pθ as a center of gravity for points at which the values of thefunction dθ(ƒ) are than greater than a threshold value.

The following description explains an embodiment for finding theastigmatism correction quantity in the whole control unit 26 frominformation on the astigmatism received from the astigmatism and focuscorrection quantity computation image processing circuit 53. When the 4directions p0, p45, p90 and p135 at angles 0 degrees, 45 degrees, 90degrees and 135 degrees respectively are used, the astigmatism and focuscorrection quantity computation image processing circuit 53 computes theastigmatism vector (dx, dy)=(p0−p90, p45−p135) and supplies theastigmatism vector to the whole control unit 26. Next, the whole controlunit 26 apportions the astigmatism vector to the astigmatism correctionquantities Δstx and Δsty in accordance with Eq. (4) as follows:Δstx=mxx∘dx+mxy∘dyΔsty=myx∘dx+myy∘dy  (4)

where mxx, mxy, myx and myy are astigmatism correction quantityapportioning parameters computed in advance on the basis of acharacteristic of the astigmatism corrector 60 and stored typically inthe storage unit 57. The astigmatism adjustment unit 64 passes on theastigmatism correction quantities Δstx and Δsty received from the wholecontrol unit 26 to the astigmatism adjustment circuit 61, requesting theastigmatism adjustment circuit 61 that the astigmatism correctionquantities Δstx and Δsty be changed by β Δstx and β Δsty where notationβ denotes a correction quantity reduction coefficient. In response tosuch a request, the astigmatism adjustment circuit 61 requests theastigmatism corrector 60 to change the astigmatism correction quantitiesΔstx and Δsty by β Δstx and β Δsty.

In the whole control unit 26, a focal point correction quantity is setat (p0+p45+p90+p135)/4 due to the fact that the focal offset z receivedfrom the astigmatism and focus correction quantity computation imageprocessing circuit 53 is an average value of focal positions in therespective directions. Thus, the astigmatism adjustment unit 64 passeson the focal point correction quantity received from the whole controlunit 26 typically to the focal position control unit 22. The focalposition control unit 22 then corrects the objective lens 18 by thefocal point correction quantity.

It should be noted that another embodiment is achievable. In thisalternative embodiment, the astigmatism and focus correction quantitycomputation image processing circuit 53 finds the astigmatism magnitudeδ=|(dx, dy)| and the astigmatism direction α=½ arctan (dy/dx) from theastigmatism vector (dx, dy) and supplies the astigmatism magnitude δ andthe astigmatism direction α to the whole control unit 26. The wholecontrol unit 26 then converts the astigmatism magnitude δ and theastigmatism direction α into astigmatism correction quantities Δstx andΔsty.

When directional sharpness magnitudes pθ in n directions are used wheren is an integer equal to or greater than 3, the astigmatism and focuscorrection quantity computation image processing circuit 53 applies asinusoidal waveform to these pieces of data to find the astigmatismmagnitude δ, the astigmatism direction α and the focal offset z from aphase, an amplitude and an offset thereof.

If the astigmatism correction quantities are changed, the focal pointmay be affected by the changes, hence, being shifted slightly. In thiscase, typically, the whole control unit 26 multiplies the astigmatismcorrection quantities Δstx and Δsty by their respective propercoefficients and uses the products obtained as changes in astigmatismcorrection quantities Δstx and Δsty.

The following description explains another embodiment of the presentinvention for correcting the astigmatism and the focal point at an evenhigher speed by referring to FIGS. 8 and 9. In this embodiment, thesurface of the calibration target 62 is inclined as shown in FIG. 8 (a)or has a staircase like shape as shown in FIG. 8 (b). Reference numerals62 a and 62 b denote the former and latter calibration targetsrespectively which each have a proper pattern created on the surfacethereof. The calibration target 62 a or 62 b is placed on the samplebase 21 as shown in FIGS. 1 and 10. Thus, a particle image of thecalibration target 62 a or 62 b which also serves as a sample has afocal point f varying from area to area. It should be noted that thedifference in height between a reference point on the calibration target62 a or 62 b and the surface of the sample 20 is measured in advance. Atechnique for automatically correcting a height for both the calibrationtarget 62 and the sample 20, and a technique for measurement using anoptical height sensor to be described later.

By using the calibration target 62 a or 62 b shown in FIG. 8 (a) or 8(b) respectively, it is possible to obtain a particle image that has afocus f varying from area to area. Thus, a flowchart shown in FIG. 9 isdifferent from the flowchart shown in FIG. 5 in that a particle imageobtained at a step S51′ of the former has a height (or a focus f)varying from area to area and includes edge components of about equalquantities in at least 3 directions. At the same step, directionalsharpness magnitudes pθ(ƒ) of such a particle image in the directionsare computed. Thereafter, the quantities for correcting the astigmatismand the focal point are found to be used in an adjustment of theastigmatism and the focal point in the same way as the steps S52 to S55of the flowchart shown in FIG. 5. In this way, by acquiring only 1particle image, the astigmatism and the focal point can be corrected ata relatively high speed.

In addition, the same effect as that of the embodiment described abovecan be obtained by using a flat surface calibration target 62 or a flatsurface sample 20. In the case of such a calibration target 62 or such asample 20, a particle image is taken while the focal point is beingchanged at a relatively high speed. In this way, it is possible toobtain an image with a focal point varying from area to area as is thecase with the embodiment described above. As a result, by acquiring only1 particle image, the astigmatism and the focal point can be correctedat a relatively high speed.

The following description explains a relation between inspections ormeasurements of a substrate and astigmatism and focal point corrections.The actual sample 20 serving as a substrate subjected to an inspectionor a measurement is mounted on the sample base 21. Then, at leastinformation on a predetermined position on the object substrate 20 issupplied to the whole control unit 26 by using an input means 59 to bestored in the whole control unit 26. The predetermined position reportedto the whole control unit 26 is a position subjected to an inspection ora measurement. The input means 59 is typically a recording medium or anetwork. Thus, when the inspection or the measurement of the objectsubstrate 20 is implemented, the XY stage 46 is controlled by a commandissued by the whole control unit 26 to take the predetermined positionon the object substrate 20 into the visual field of the charged particleoptical system. Subsequently, a charged particle beam is radiated ontothe surface of the object substrate 20 to carry out a scanning operationand a particle image obtained as a result of the radiation is detectedby the particle detector 16. The particle image is then subjected to A/Dconversion before being stored in the image memory 55. Then, theinspection and measurement image processing circuit 56 processes theparticle image in order to carry out the inspection or the measurement.At that time, at the position subjected to the inspection or themeasurement, the astigmatism and the focal position are corrected byusing the method provided by the present invention. As a result, it ispossible to carry out an inspection or a measurement based on a particleimage with the astigmatism thereof always corrected.

In the case of an inspection and measurement apparatus provided withtypically an optical height detection sensor 13 wherein the objectsubstrate is affected by electric charge, dirt, a damage or the likeonly slightly, a converged charged particle beam is radiated to thesample 20 in a scanning operation for an inspection or a measurementinstead of using a height detected by the optical height detectionsensor 13 at each position on the sample 20 subjected to the inspectionor the measurement as feedback information in the processing of thefocal point and radiating the converged charged particle beam in ascanning operation for an adjustment of the focal point and theastigmatism. Thus, the effect of electric charge, dirt, a damage or thelike only on the object substrate 20 or the sample 20 can be suppressedto a minimum. In this case, the astigmatism and the focal point areautomatically adjusted by using a separate position on the sample 20 orthe calibration target 62 provided on the sample base 21. The automaticadjustment is carried out in advance or periodically in the course ofthe inspection or the measurement. Further, the calibration target 62can be a sample having an inclined or staircase like surface shown inFIG. 8 or a sample having a flat surface as shown in FIG. 1.

In the automatic adjustment of the astigmatism and the focal pointaccording to the present invention as described above, shifts of thefocal point and the astigmatism which occur with the lapse of time arecorrected. In the automatic adjustment of the astigmatism and the focalpoint according to the present invention, however, it is necessary toadjust a detected offset with the optical height detection sensor 13 inadvance. Differences in height or variances in height among positions onthe actual sample 20 or the object substrate 20 are detected by theoptical height detection sensor 13 in the correction of the focal point.Thus, only during an inspection or a measurement is an astigmatism freeconverged charged particle beam radiated to the actual sample 20 in anin focus state in a scanning operation. Therefore, a particle image canbe detected in a state where the effect of electric charge, dirt, adamage or the like only on the actual sample 20 can be suppressed to aminimum. As a result, the object substrate 20 can be inspected ormeasured with a relatively high degree of precision.

In addition, when it is desired to calibrate not only an offset betweenthe optical height detection sensor 13 and the focal position controlunit 22, but also the gain, a plurality of calibration targets 62 withtheir heights known are provided in advance. Then, an automaticcorrection of the focal point and a detection by using the opticalheight detection sensor 13 are both carried out on each of thecalibration targets 62 in order to calibrate the gain and evenlinearity. Furthermore, an automatic correction of the focal point and adetection by using the optical height detection sensor 13 can both becarried out on each of the calibration targets 61 or the samples 20while the height of the calibration target 61 or the sample 20 is beingvaried by using the Z shaft of the XY stage 46 in order to calibrate thegain and even the linearity.

Moreover, while the XY stage 46 is being moved continuously in thehorizontal direction as shown in FIG. 10, the beam deflector 15 isdriven to radiate a converged charged particle beam in a scanningoperation in a direction crossing the moving direction of the XY stage46 almost perpendicularly in particular to allow the particle detector16 to continuously detect a particle image. In an inspection or ameasurement carried out at a relatively high speed, the controldescribed below is executed. A height detected by the optical heightdetection sensor 13 is fed back to the focal position control unit 22and the deflection control unit 47 all the time. By doing so, while ashift of the focal point and a rotation of the deflection are beingcorrected all the time, a particle image is detected. It is thuspossible to implement a relatively high speed inspection or a relativelyhigh speed measurement with a relatively high degree of precision overthe entire surface of the actual sample 20. It should be noted that,instead of driving the focal position control unit 22 to correct thefocal point, the Z shaft of the XY stage 46 can of course be driven toresult in the same effects. In the mean time, the operation is shiftedto the calibration target 62 periodically as shown in FIG. 10 toautomatically correct the focal point and the astigmatism. It is thuspossible to carry out a relatively high precision and high sensitivityinspection or a relatively high precision and high sensitivitymeasurement based on a particle image with the focal point and theastigmatism thereof corrected with a relatively high degree ofprecision.

The following description explains a method of finding the centerposition of an area under a curve representing the directional sharpnessmagnitude wherein a function having a peak such as a quadratic functionor a Gaussian function is used to represent the curve with reference toFIG. 11. As shown in the figure, a point at which a maximum value of thesharpness magnitude is located is found. Then, a convex function such asa quadratic function or a Gaussian function is applied to N data pointspreceding and succeeding the maximum value point. For N=3, it ispossible to find such parameters that all the data points are placed onthe curve of the quadratic function or the Gaussian function. Thus, thecenter position of an area under the curve representing the directionalsharpness magnitude can be found by interpolation.

By a maximum position or interpolation based on a maximum valueposition, however, an error will be generated particularly in the caseof a large magnitude of astigmatism. This problem is shown in FIG. 12.Consider a sharpness magnitude in the 0 degree direction in a case inwhich astigmatism is generated in the approximately ±45 degreedirection. Thus, if the spot of the charged particle beam is in an infocus state in the ±45 degree direction, the spot cross section lengthin the 0 degree direction is narrowed. In an in focus state, on theother hand, the spot cross section length in the 0 degree direction iswidened. Generally, the narrower the spot cross section length in the 0degree direction, the greater the sharpness magnitude. Thus, a sharpnessquantity curve in a direction with no generated astigmatism tends toexhibit a double peaked characteristic for a large sharpness magnitudeas is the case with d0 (f) and d90 (f) curves shown in FIG. 12 (b). Ifan interpolation based only on a maximum value position is adopted insuch a case, a one sided position like a point B shown in FIG. 12 (c)will be determined to be the center position of the area under the curverepresenting the directional sharpness magnitude. In this case, a valueclose to the maximum p45 of the function d45 (f) is taken as the centerof the area under the curve representing the function d0 (f). In theexample of FIG. 12, when maximum position is used, p0 becomes very closeto p45 and p90 becomes very close to p135. In this case, the estimated±45 degree component of the astigmatic focus distance becomes twice aslarge as it should be. If this astigmatic focus distance is used for theastigmatic correction, the astigmatism in this direction is correctedtoo much, resulting in unstable behavior. In contrast, depending on thetechnique of searching for a maximum value, a point C representing themaximum value may be taken as the center of the area under the curverepresenting the function d0 (f). In this case, almost no correctionoccurs for astigmatism in the ±45 degree direction. In order to find theastigmatism magnitude and the astigmatism direction correctly asexplained earlier by referring to FIG. 6, however, it is necessary todetermine a middle point between the points B and C such as a point Ashown in FIG. 12 (c) as the center of the area under the curverepresenting the function d0 (f).

As described above, in select embodiments according to the presentinvention, a point between the points B and C is found as the center ofthe area under the curve representing the astigmatism magnitude independence on the sizes of the peaks of the points B and C. There are avariety of techniques of determining such a middle point. Someembodiments implementing these techniques are explained as follows. Itshould be noted that the scope of the present invention is not limitedto the described embodiments. That is, the scope of the presentinvention includes the use of any technique to find a middle value inaccordance with the sizes of the peaks.

FIG. 13 is an explanatory diagram showing a technique based on thecenter of gravity. According to this technique, a maximum value isfound. Then, the maximum value is multiplied by a constant, α, equal toor smaller than 1, resulting in a product that can be used as athreshold value. The center of gravity of an area enclosed by a segmentof a curve and a line representing the threshold value is thendetermined, where the curve represents variations in directionalsharpness magnitude with the focal point position and the segmentrepresents points on the curve each having a value greater than thethreshold value. The center of gravity is used as the center of the areaunder the curve representing the directional sharpness magnitude.

That is, the center pθ of the area under the curve representing thedirectional sharpness magnitude is found in accordance with thefollowing equation:

${p\;\theta} = \frac{\sum{f \cdot \left( {{d\;{\theta(f)}} - {\alpha \cdot {MaximumValue}}} \right)}}{\sum{d \cdot \left( {{d\;{\theta(f)}} - {\alpha \cdot {MaximumValue}}} \right)}}$

FIG. 14 is a diagram showing a technique based on a weighted average. Ifthere are a plurality of maximum values for a directional sharpnessmagnitude, peak positions of the maximum values are found and a weightedaverage value of the peak positions is computed with weights determinedin accordance with the heights of the maximum value points at the peakpositions. The weighted average is used as the center of the area underthe curve representing the directional sharpness magnitude. Letnotations B and C denote the positions of maximum values. In this case,the center pθ of the area under the curve representing the directionalsharpness magnitude is computed in accordance with the followingequation:

${p\;\theta} = \frac{\left( {{d\;{{\theta(c)} \cdot B}} + {d\;{{\theta(B)} \cdot C}}} \right)}{\left( {{d\;{\theta(C)}} + {d\;{\theta(B)}}} \right)}$

FIG. 15 is a diagram showing a technique based on symmetry matching. Inaccordance with this technique, variations in degree of matching with pθare found. The degree of matching represents coincidence between a curvedθ(ƒ) representing variations in directional sharpness magnitude withthe focal point position and a curve dθ(a−ƒ) of image inversionsymmetrical with respect to an axis of symmetry f=a on the left andright sides of the axis of symmetry f=a. The position a of an axis ofsymmetry providing the highest degree of coincidence is determined asthe in focus position pθ. As a degree of coincidence, a point of amaximum correlation value can also be used. As an alternative, a pointproviding a minimum sum of squares of differences can also be used as adegree of coincidence. Many other indicators generally used as anindicator of the degree of coincidence can be used. The embodimentsdescribed above are used for exemplifying a case in which a chargedparticle beam apparatus is applied to an inspection and measurementapparatus.

It should be noted, however, that the techniques described herein withrespect to the example of a charged particle beam apparatus can also beapplied to other equipment such as a fabrication apparatus using acharged particle beam.

The preceding has been a description of the preferred embodiment of theinvention. It will be appreciated that deviations and modifications canbe made without departing from the scope of the invention, which isdefined by the appended claims.

The preceding has been a description of the preferred embodiment of theinvention. It will be appreciated that deviations and modifications canbe made without departing from the scope of the invention, which isdefined by the appended claims.

1. A charged particle beam apparatus comprising: a stage for mounting aspecimen, wherein the stage is movable at least in one direction; acalibration target fixed to the stage, wherein plural patterns having anedge component in at least three directions are formed on a surface ofthe calibration target; a charged particle optical unit for converging acharged particle beam emitted by a charged particle source; a scanningunit for irradiating and scanning the converged charged particle beam onthe specimen and the calibration target; a focal point control unit forcontrolling a focal point of the converged charged particle beam; anastigmatism adjustment unit for adjusting astigmatism of the convergedcharged particle beam; an image detection unit for obtaining an image ofthe specimen by detecting particles generated from the specimen by theirradiating of the converged charged particle beam; an image processingunit for processing the image obtained by the image detection unit; anda control system for adjusting and controlling the astigmatism of theconverged charged particle beam by computing an astigmatism correctionquantity and a focal point correction quantity based on a focal offsetand astigmatism of the converged charged particle beam obtained from aplurality of images of the calibration target by scanning thecalibration target with the converged charged particle beam using thescanning unit, obtaining an image of the calibration target using theimage detection unit and processing the obtained image of thecalibration target using the image processing unit.
 2. A chargedparticle beam apparatus according to claim 1, further comprising aheight detection unit for optically detecting a height of a surface ofthe specimen mounted on the stage.
 3. A charged particle beam apparatusaccording to claim 2, wherein the height detection unit detects a heightof the surface of the specimen while moving the stage at least in onedirection.
 4. A charged particle beam apparatus according to claim 3,wherein the control system controls the focal point control unit tocontrol the focal point of the converged charged particle beam while thestage is moving at least in one direction using a height of the surfaceof the specimen detected by the height detection unit.
 5. A chargedparticle beam apparatus according to claim 1, wherein the imagedetection unit detects a plurality of images at different focal pointpositions by controlling the focal point control unit.
 6. A method fordetecting an image of a specimen, the method comprising: setting aspecimen on a table, wherein a calibration target is fixed on a surfaceof the table, the table being movable at least in one direction;converging a charged particle beam emitted from a charged particlesource; irradiating and scanning the converged charged particle beamonto a surface of the calibration target on which plural patterns havingan edge component in plural directions are formed; detecting a pluralityof images of the calibration target by detecting particles generatedfrom the specimen by the irradiating and scanning of the convergedcharged particle beam by changing focal positions of the convergedcharged particle beam; computing an astigmatism correction quantity anda focal point correction quantity based on a focal offset andastigmatism of the converged charged particle beam obtained byprocessing the plurality of detected images; adjusting the astigmatismof the converged charged particle beam based on the computed astigmatismcorrection quantity; adjusting the focal point of the converged chargedparticle beam based on the computed focal point correction quantity; andirradiating and scanning the converged charged particle beam onto asurface of the specimen; obtaining an image of the specimen by detectingparticles generated from the specimen by the irradiating and scanning ofthe converged charged particle beam; and while irradiating and scanningthe converged charged particle beam onto the surface of the specimen,periodically repeating the step of irradiating and scanning theconverged charged particle beam onto a surface of the calibration targetto the step of adjusting the focal point of the converged chargedparticle beam.
 7. A method according to claim 6, further comprisingoptically detecting a height of the surface of the specimen which ismoving at least in one direction by moving the table, wherein thesurface of the specimen is irradiated and scanned by the convergedcharged particle beam.
 8. A method according to claim 6, wherein thefocal offset of the converged charged particle beam is obtained fromdirectional sharpness magnitudes for at least three directions for aplurality of focal positions from the plurality of detected images ofthe calibration target.
 9. A method according to claim 8, wherein theastigmatism of the converged charged particle beam is obtained from arelationship between the focal offset and the directional sharpnessmagnitudes.
 10. A method for detecting an image of a specimen, themethod comprising: setting a specimen on a table, wherein a calibrationtarget is fixed on a surface of a table, the table being movable atleast in one direction; irradiating and scanning a converged electronbeam onto a surface of the calibration target on which plural patternshaving at least three edge components in three directions are formed;detecting a plurality of images of the calibration target by detectingsecondary electrons generated from the specimen by the irradiating andscanning of the converged electron beam by changing focal positions ofthe converged electron beam; computing an astigmatism correctionquantity and a focal point correction quantity based on a focal offsetand astigmatism of the converged electron beam obtained by processingthe plurality of detected images of the plural patterns; controlling afocal point of the converged electron beam using the computed focalpoint correction quantity; irradiating and scanning the convergedelectron beam onto a surface of the specimen; and obtaining an image ofthe specimen by detecting secondary electrons generated from thespecimen by the irradiating and scanning of the converged electron beam.11. A method according to claim 10, further comprising opticallydetecting a height of the surface of the specimen.
 12. A methodaccording to claim 11, further comprising moving the table in onedirection while irradiating and scanning the converged electron beamonto the surface of the specimen and optically detecting the height ofthe surface of said specimen.
 13. A method according to claim 10,wherein the focal point correction quantity is obtained from directionalsharpness magnitudes for at least three directions for a plurality offocal positions from the plurality of detected images.